Method to prevent defects on SRAM cells that incorporate selective epitaxial regions

ABSTRACT

An SRAM device and method of forming MOS transistors of the device having reduced defects associated with selective epitaxial growth in moat tip regions is discussed. The SRAM device comprises a core region and a logic region, logic transistors within the logic region of the SRAM, and selective epitaxial regions grown on both source and drain regions; and memory cell transistors within the core region of the SRAM, and having the selective epitaxial regions grown on only one of the source and drain regions. One method of forming the MOS transistors of the SRAM cell comprises forming a gate structure over a first conductivity type substrate to define a channel therein, masking one of the source and drain regions in the core region, forming a recess in the substrate of the unmasked side of the channel, epitaxially growing SiGe in the recess, removing the mask, and forming the source and drain extension regions in source/drain regions.

FIELD OF INVENTION

The present invention relates generally to semiconductor devices andmore particularly to static random access memory (SRAM) devices andassociated methods of manufacture having reduced defects associated withselective epitaxial growth in moat tip regions, and improved low leakagetransistors having improved mobility due to channel strain.

BACKGROUND OF THE INVENTION

A conventional MOS transistor generally includes a semiconductorsubstrate, such as silicon, having a source, a drain, and a channelpositioned between the source and drain. A gate stack composed of aconductive material (a gate conductor), an oxide layer (a gate oxide),and sidewall spacers, is typically located above the channel. The gateoxide is typically located directly above the channel, while the gateconductor, generally comprised of polycrystalline silicon (poly orpolysilicon) material, is located above the gate oxide. The sidewallspacers protect the sidewalls of the gate conductor and localize thesource and drain dopants relative to the gate conductor.

Generally, for a given electric field across the channel of a MOStransistor, the amount of current that flows through the channel isdirectly proportional to a mobility of carriers in the channel. Thus thehigher the mobility of the carriers in the channel, the more current canflow and the faster a circuit can perform when using high mobility MOStransistors. One way to increase the mobility of the carriers in thechannel of an MOS transistor is to produce a mechanical stress in thechannel.

A compressive strained channel has significant hole mobility enhancementover conventional devices. A tensile strained channel achievessignificant electron mobility enhancement. The most common method ofintroducing strain in a silicon channel region is to recess the sourceand drain regions adjacent to the channel and to fill the recessedregions with a stressor material by selective epitaxial growth.

A compressive strain in the channel is obtained when the stressormaterial epitaxially grown is SiGe. Similarly, tensile strain in thechannel is obtained when the stressor material is Si:C.

SRAM devices, for example, use a mixture of such MOS transistors on anintegrated circuit chip including carefully tailored and matchedtransistors utilized in the memory array (core region) as well asgeneral purpose logic transistors used in various peripheral controlcircuits (logic region) which control access to the array. However,advanced SRAM devices have a tendency to produce various defectsassociated with selective epitaxial growth in moat tip regions. Thesedefects are due to the reduced dimensions of the region, preventingproper epitaxial growth and result in extremely thin epitaxial layers.Such defects also result in degraded electrical characteristics,degraded contacts and silicide formation, and poor diodecharacteristics. In addition, transistors fabricated with selectiveepitaxial regions in the source/drains suffer from high off-statecurrent due to sub-surface leakage.

It would be advantageous to have an SRAM device and method thateffectively and reliably provides strain to the device in order toimprove carrier mobility, while reducing defects associated withselective epitaxial growth in moat tip regions of the core region.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the invention. This summary isnot an extensive overview of the invention, and is neither intended toidentify key or critical elements of the invention, nor to delineate thescope thereof. Rather, the primary purpose of the summary is to presentsome concepts of the invention in a simplified form as a prelude to themore detailed description that is presented later.

The present invention relates to a device and method of fabrication,wherein the MOS transistors, for example, generally PMOS transistors ofan SRAM memory device having core and logic regions, exhibits improvedmobility or drive current due to the application of stress to thechannel, and reduced defects associated with selective epitaxial growthin moat tip regions of the core region. The MOS transistors have astress application region formed in the semiconductor body near thechannel region; consequently, less stress inducing species is needed toobtain the desired strain within the channel, thereby providing improvedcarrier mobility without a substantial number of threading dislocationdefects. In the fabrication of the SRAM device, the moat tip regions ofthese PMOS transistors are protected from etching and selectiveepitaxial growth in the moat tip regions that otherwise may causethinning of the epitaxial growth in the source or drain regionsneighboring the shallow trench isolation (STI) areas.

In particular, PMOS transistors of the SRAM memory cells that havesource and drain regions adjacent to STI regions (generallycorresponding to moat tip regions) are masked with an epitaxial etchmask to avoid the etching and selective epitaxial growth in theseregions neighboring the STI structures. The stress inducing regions inthe semiconductor body are formed by selective epitaxial deposition of asilicon germanium material within recesses formed in the body. Therecesses are formed in the body self-aligned to the edges of offsetspacers and prior to the formation of the sidewall spacers employed forsource/drain implants. Consequently, the recesses are very close to thechannel and strain associated with the silicon germanium materialtherein is more effectively translated to the channel.

In accordance with one aspect of the present invention, an asymmetricSRAM memory device having MOS transistors in logic and core regions,comprises one or more logic transistors having a semiconductor body of afirst conductivity type, and selective epitaxial region grown on bothsource and drain regions of the logic transistor. The SRAM devicefurther comprises one or more memory cell transistors within the coreregion having a semiconductor body of the first conductivity type, andhaving selective epitaxial regions grown on only one of the source anddrain regions of the cell transistors, the other one of the source anddrain regions generally corresponding to moat tip regions.

In accordance with another aspect of the present invention, a method ofasymmetrically forming a MOS transistor is provided, wherein a gatestructure is formed over a semiconductor body, thereby defining achannel region therebelow. A mask (e.g., an epitaxial etch mask, usedprior to etch processing and selective epitaxial growth) is patternedover one of a source and drain regions on opposing sides of the channelof the MOS transistor, for example, in one embodiment of the invention,the masked region generally corresponding to a moat tip region. A recessis formed in the body that is self-aligned with respect to the gatestructure in the other unmasked one of the source and drain regionscorresponding to the unmasked side of the channel. In one example, therecess is formed immediately after mask patterning or after formation ofoffset spacers (used for extension region implants) on lateral edges ofthe gate. The recess is then filled with silicon germanium via aselective epitaxial deposition process. The silicon germanium materialhas different lattice spacing than the silicon channel, therebyimparting a compressive strain to the channel region under the gate. Themask is removed and the source and drain extension regions are formed ofa second conductivity type in the semiconductor body in the source anddrain regions. Thereafter, conventional back-end processes may beutilized.

Similarly, in another aspect of the present invention, a method ofasymmetrically forming MOS transistors of an SRAM memory device having alogic and core region comprises, forming a gate structure over a firsttype semiconductor body of the MOS transistors in both logic and coreregions of the SRAM device. A mask (e.g., an epitaxial etch mask, usedprior to etch processing and selective epitaxial growth) is patternedover both source and drain regions on opposing sides of the channel ofthe MOS transistors in the logic region. A mask is patterned over one ofthe source and drain regions on opposing sides of the channel of the MOStransistors in the core region, the masked region generallycorresponding to a moat tip region. Recesses are formed in the body thatare self-aligned with respect to the gate structure in the unmaskedsource and drain regions. In one example, the recess is formedimmediately after mask patterning or after formation of offset spacers(used for extension region implants) on lateral edges of the gate. Therecess is then filled with silicon germanium via a selective epitaxialdeposition process. The mask is removed and the source and drainextension regions are formed of a second conductivity type in thesemiconductor body in the source and drain regions. Thereafter,conventional back-end processes may be utilized.

In one aspect of the invention, the first conductivity typesemiconductor body of the MOS transistor comprises PMOS transistors of a6T SRAM cell.

In one example, the silicon germanium material is doped with boronin-situ for the formation of p-type extension regions. In anotherexample, an extension region implant is performed into the silicongermanium material after the selective epitaxial deposition, followed bythe formation of sidewall spacers and source/drain implants.

The following description and annexed drawings set forth in detailcertain illustrative aspects and implementations of the invention. Theseare indicative of but a few of the various ways in which the principlesof the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary semiconductor device thatincorporates SRAM memory circuitry such as may be used in accordancewith one or more aspects of the present invention, the device havingcore and logic regions;

FIG. 2A is a schematic diagram of a conventional 6T static random accessmemory (SRAM) cell;

FIGS. 2B-2E are top plan and corresponding cross-sectional side viewsillustrating a conventional epitaxial etch mask pattern that determineswhich regions will be etched in a substrate of the SRAM cell of FIG. 2A,and including some typical defects that may result during subsequentselective epitaxial growth thereon in such regions;

FIG. 3A is a schematic diagram of another 6T static random access memory(SRAM) cell such as may be used in accordance with the presentinvention;

FIGS. 3B and 3C are top plan and a corresponding cross-sectional sideview illustrating an exemplary modified epitaxial etch mask pattern usedin accordance with the present invention that prevents etching andsubsequent selective epitaxial growth from the source/drain regions thatgenerally correspond to moat tip regions of the SRAM cell of FIG. 3A;

FIGS. 4A-4C are additional top plan and a corresponding cross-sectionalside views of two exemplary SRAM cells of an array of cells, similar tothe SRAM cell of FIGS. 3A-3C, illustrating the exemplary modifiedepitaxial etch mask pattern used in accordance with the presentinvention that prevents etching and subsequent selective epitaxialgrowth from the source/drain regions generally corresponding to moat tipregions;

FIGS. 5A-5C are more top plan and a corresponding cross-sectional sideviews of four exemplary SRAM cells of an array of cells, similar to theSRAM cell of FIGS. 3A-3C, illustrating the exemplary modified epitaxialetch mask pattern used in accordance with the present invention thatprevents etching and subsequent selective epitaxial growth from thesource/drain regions generally corresponding to moat tip regions;

FIG. 6 is a flow diagram illustrating an exemplary method of forming anasymmetric SRAM memory device having improved mobility while reducingdefects associated with selective epitaxial growth in moat tip regionsof the core region of the memory device according to one aspect of thepresent invention;

FIGS. 7A-7I are fragmentary cross section diagrams illustrating varioussteps of forming PMOS transistors, such as may be used in the coreregion of an SRAM device in accordance with the method of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

One or more implementations of the present invention will now bedescribed with reference to the attached drawings, wherein likereference numerals are used to refer to like elements throughout, andwherein the illustrated structures are not necessarily drawn to scale.The invention provides transistor and SRAM memory structures and methodsin which transistor mobility or drive current is improved whileminimizing defects associated with selective epitaxial growth in moattip regions of the core region of such memory devices.

Referring now to FIG. 1, an exemplary semiconductor device thatincorporates SRAM memory circuitry 100 is illustrated such as may beused in accordance with one or more aspects of the present invention.The SRAM memory containing device 100 comprises, for example, asemiconductor integrated circuit chip having a core region 110 and alogic region 120. The core region 110 comprises an array of SRAM memorycells, for example, an array of 6T SRAM memory cells such as SRAM cell200 of FIGS. 2A-2E. The logic region 120 comprises various memorycontrol and logic functions associated with accessing, reading, andwriting operations of the memory array 110 as well as general logic forthe device.

FIG. 2A illustrates a schematic diagram of an exemplary 6T static randomaccess memory (SRAM) cell 200 such as may be used in accordance with thesystem and methods of the present invention.

FIGS. 2B, 2C, 2D, and 2E are top plan and corresponding cross-sectionalside views, respectively, illustrating conventional epitaxial etch maskpatterns that determines which regions will be etched in a substrate ofthe 6T SRAM cell 200 of FIG. 2A, while FIG. 2E further illustrates sometypical defects that may result during subsequent selective epitaxial(epi) growth thereon in such regions. The dot-dash line running throughFIG. 2B labeled A-A, represents a break in the composite cross sectionA-A, as shown in FIG. 2C, and the dot-dash line running through FIG. 2Blabeled B-B, represents a break in the composite cross section B-B, asshown in FIGS. 2D and 2E.

FIG. 2A, for example, illustrates a typical SRAM cell 200, whichgenerally comprises a pair of cross-coupled inverters 202, 204,generally connected between Vdd 212 and Vss 214 to store a data bitstate. SRAM cell 200 further comprises a pair of pass transistors;including pass gate A 216 and pass gate B 218 to read and write a databit between the cross-coupled inverters; inverter A 202 and inverter B204, and bitline BL 230 and bitline-bar BLB 232, respectively.Respective inverters 202, 204 comprise p-type MOS (PMOS) pull-up or loadtransistors; load transistor A 220 and load transistor B 222, and ann-type (NMOS) pull-down or driver transistor A 224 and driver transistorB 226. Pass gate transistors 216, 218 are generally n-type as well,generally having shared gate leads connected together, and controlled bythe same wordline WL 238.

The illustrated exemplary SRAM cell comprises six transistors and istermed a 6T full CMOS SRAM cell. When the channels of all thetransistors are formed in the single crystal silicon substrate, it iscalled a single crystalline bulk CMOS 6T SRAM cell. It is also referredto as a planar SRAM cell when all the transistors are made in the samesubstrate material (e.g., bulk crystalline silicon, SOI, etc.).

FIGS. 2B and 2C, for example, illustrates a layout of an SRAM array 240of two such SRAM memory cells 200 and 201, during an epitaxial (epi)etch mask 242 patterning step of a fabrication process. SRAM cells 200and 201 of array 240 are arranged adjacent to each other havinghorizontal rows of polysilicon gate structures 244 and sharing verticalcolumns of NMOS active regions 246 and PMOS active regions 247, andhaving isolation structures, for example, shallow trench isolationregions (STI) 251 therebetween to separate NMOS transistors formed inthe active regions 246 from the PMOS transistors formed in the activeregions 247. In the illustrated fabrication step, the epitaxial etchmask 242 is patterned overlying the NMOS active regions 246, therebyforming unmasked openings 242 a in the mask 242 having a thickness 242 bthat exposes the PMOS active regions 247 to a subsequent etch processand to a selective epitaxial growth in these unmasked openings 242 a.

In general, SRAM cells are more stable and have better data retentionwherein the respective PMOS (220, 222) transistors and NMOS (224, 226)transistors are matched for the two inverters (202, 204). However, asdimensions are continually reduced to scale down devices, advanced SRAMdevices have a tendency to produce defects associated with the selectiveepitaxial growth in moat tip regions 252.

For example, FIGS. 2D and 2E illustrate a cross section B-B of two PMOSload B transistors 222 of the two adjacent cells 200 and 201 sharing asingle n-well 257 and a common source/drain region 260. In particular,the shared source/drain region 260 is the drain connection to the Vdd212 drain terminal. The other outside source/drain regions 264 adjacentto the STI structures STI 251, are ideally etched and grown in theselective epitaxial etch and growth processes to provide a uniform fullthickness 260 b of FIG. 2D. It will be appreciated that the figures ofcross sections B-B may also represent two PMOS load A transistors 220.FIGS. 2D and 2E further illustrate details of a gate structure of loadtransistor load B 222, comprising a gate oxide layer 262 formedoverlying the n-doped silicon of the n-well 257, a gate polysiliconstructure 244 overlying the gate oxide 262, and side wall spacers 268formed on lateral sidewalls of the gate for self-aligning subsequentsource/drain implant processes in the source/drain regions 260 and 264.

However, because of the continued scaling of dimension in these advancedSRAM devices, such proper epitaxial growth is often inhibited in thesemoat tip regions 252 and results in extremely thin epitaxial growthdefect area 282 in the source/drain regions 264 adjacent to isolationstructures STI 251. As shown by defect area 282 in FIG. 2E, the SiGegrowth is slowed or inhibited by the proximity to the isolationstructure STI 251, thus the SiGe growth in the source/drain region 264assumes a tapered shape which gradually regains nearly full depth 264 bfurther from the STI 251.

Such defects also result in degraded electrical characteristics of thetransistors of the SRAM cells 200, 201, degraded contacts and silicideformation, and poor diode characteristics. Another defect that may beproduced in the moat tip regions 252, occurs because of an increasedpropensity for silicide pipe formation 285. The silicide pipe 285 maycomprise a nickel silicide formed during a silicidation process justbelow a triple point created by the thinned source/drain region 264, theSTI 251, and the n-well 257. The silicide pipe 285 tends todetrimentally short these regions to the transistor channel and to thesubstrate. In addition, transistors fabricated with selective epitaxialregions grown at both source/drains 260 and 264 suffer from highoff-state current due to sub-surface leakage (e.g., in the n-well 257)because these sub-surface regions are further from the control of thegate.

Accordingly, the solution of the present invention, is to protect ormask the moat tip regions during the entire epitaxial loop, whichcomprises an etch process for recess formation and the selectiveepitaxial growth within the recesses. Thus, those areas which are maskedwill not receive the recess etch or the epitaxial growth of, forexample, silicon germanium (SiGe), Boron doped SiGe, or Carbon dopedsilicon.

FIGS. 3A, 3B and 3C illustrate another 6T static random access memory(SRAM) cell 300 such as may be used in accordance with the presentinvention. SRAM cell 300 is similar to that of cell 200 of FIG. 2A, andas such need not be described again for the sake of brevity.

FIGS. 3B and 3C illustrate the physical layout of the SRAM cell 300 ofFIG. 3A further illustrating an exemplary modified epitaxial etch maskpattern 342 and openings 342 a, such as may be used in accordance withthe present invention to prevent etching and subsequent selectiveepitaxial growth from the source/drain regions that generally correspondto moat tip regions of the SRAM cell 300.

FIG. 3A, illustrates an exemplary SRAM cell 300, which generallycomprises a pair of cross-coupled inverters 302, 304, generallyconnected between Vdd 312 and Vss 314 to store a data bit state. SRAMcell 300 further comprises a pair of pass transistors; including passgate A 316 and pass gate B 318 to read and write a data bit between thecross-coupled inverters; inverter A 302 and inverter B 304, and bitlineBL 330 and bitline-bar BLB 332, respectively. Respective inverters 302,304 comprise p-type MOS (PMOS) pull-up or load transistors; loadtransistor A 320 and load transistor B 322, and an n-type (NMOS)pull-down or driver transistor A 324 and driver transistor 326. Passgate transistors 316, 318 are generally n-type as well, generally havingshared gate leads connected together, and controlled by the samewordline WL 338.

FIGS. 3B and 3C, for example, illustrates a layout of an SRAM memorycell 300 during an epitaxial (epi) etch mask 342 patterning step of afabrication process. As before, SRAM cell 300 is arranged havinghorizontal rows of polysilicon gate structures 344 and vertical columnsof NMOS active regions 346 and PMOS active regions 347, and havingisolation structures, for example, shallow trench isolation regions(STI) 351 therebetween to separate NMOS transistors formed in the activeregions 346 from the PMOS transistors formed in the active regions 347.In the illustrated fabrication step, the epitaxial etch mask 342 ispatterned overlying the NMOS active regions 346, thereby formingunmasked openings 342 a in the mask 342 having a thickness 342 b thatexposes the PMOS active regions 347 to a subsequent etch process and toa selective epitaxial growth in these unmasked openings 342 a.

However, in the modified selective epitaxial etch mask patterning342/342 a of the 6T SRAM cell 300 of the present invention, additionalmask material 342 is used between the gate structures 344 and overlyingthe PMOS active regions 347 that generally correspond to moat tipregions 352. For example, these moat tip regions 352 are physicallylocated at the ends of the PMOS active region 347 formed within ann-well 357 and where the source/drain regions of load transistors 320and 322 are situated adjacent to an isolation structure STI 351. Bymasking this moat tip region 352, no etching or selective epitaxialgrowth will take place in these regions, and the defects associatedtherewith are avoided.

Thus, this solution creates improved asymmetric PMOS load transistors320, 322, wherein this masked region 342 generally corresponds to themoat tip regions 352 of the SRAM cell 300. That is, although the PMOSload transistors 320, 322 have a source/drain region on opposing sidesof a gate structure and channel region, only one of these source/drainregions receives the selective epitaxial growth, thereby asymmetricallyforming PMOS load transistors. Beneficially, this solution of thepresent invention provides a method to significantly reduce theoff-current (leakage) of the PMOS transistors (e.g., 320, 322) of theSRAM cell 300, for example, in the core region 110 of an SRAM device 100having core 110 and logic regions 120. Further, by maintaining theselective epitaxial growth on one side of the PMOS transistor, the drivecurrent is still improved greatly compared to a PMOS transistor notutilizing selective epitaxial growth.

The logic transistors of the logic region 120 of the SRAM device 100 mayreceive the conventional selective epitaxial etch and growth in bothsource and drain regions of the logic transistors, or may receive theasymmetrical masking in one of the source and drain regions.Alternately, the entire SRAM core region 110 may be excluded from theepitaxial loop.

FIGS. 4A, 4B and 4C illustrate an SRAM array 400 of two SRAM memorycells 300 and 301, during a fabrication process wherein an epitaxialetch mask 342 is patterned similar to that which was described for theSRAM cell 300 of FIGS. 3A-3C, in accordance with the present invention.Again, the exemplary modified epitaxial etch mask pattern used inaccordance with the present invention prevents etching and subsequentselective epitaxial growth in one of the source/drain regions 364generally corresponding to moat tip regions 352. The dot-dash linerunning through FIG. 4A labeled A-A, represents a break in the compositecross section A-A, as shown in FIG. 4C, and the dot-dash line runningthrough FIG. 4A labeled B-B, represents a break in the composite crosssection B-B, as shown in FIGS. 4B and 4B′.

FIG. 4B, for example, further illustrates the placement of the epitaxialetch mask 342 overlying the moat tip regions 352. The mask 342 thusprotects the source/drain region 364 in PMOS active regions 347 formedwithin the n-well 357 at the moat tip regions 352, thereby avoiding thedefects previously discussed. FIG. 4B further illustrates details of agate structure of load transistor load B 322, comprising a gate oxidelayer 362 formed overlying the p-doped silicon of the n-well 357, thegate poly 344 overlying the gate oxide 362, and side wall spacers 368formed on lateral sidewalls of the gate for self-aligning subsequentsource/drain implant processes in the source/drain regions 360 and 364.

FIGS. 5A, 5B and 5C illustrate an SRAM array 500 of four SRAM memorycells 300, 301, 302, and 303, during a fabrication process wherein anepitaxial etch mask 342 is patterned similar to that which was describedfor the SRAM cell 300 and 301 of FIGS. 4A-4C, in accordance with thepresent invention. FIG. 5A further illustrates that the exemplarymodified epitaxial etch mask pattern used over cells 302 and 303 ofarray 500 forms a mirror image of the mask over cells 300 and 301,wherein the etch mask 342 prevents etching and subsequent selectiveepitaxial growth from the source/drain regions 364 generallycorresponding to moat tip regions 352 of the array 500.

7A-7I are fragmentary cross section diagrams illustrating various stepsof forming PMOS transistors, such as may be used in the core region ofan SRAM device in accordance with the method of FIG. 6.

Referring now to FIGS. 6 and 7A-7I, further aspects of the inventionrelate to methods of fabricating MOS transistors of an SRAM memorydevice, wherein FIG. 6 illustrates an exemplary method 600 in accordancewith the invention, and FIGS. 7A-7I illustrate the exemplary MOStransistors of the SRAM memory device at various stages of fabricationin accordance with the invention. While the exemplary method 600 isillustrated and described below as a series of acts or events, it willbe appreciated that the present invention is not limited by theillustrated ordering of such acts or events. For example, some acts mayoccur in different orders and/or concurrently with other acts or eventsapart from those illustrated and/or described herein, in accordance withthe invention. In addition, not all illustrated steps may be required toimplement a methodology in accordance with the present invention.Furthermore, the methods according to the present invention may beimplemented in association with the fabrication of SRAMs, ICs andcomposite transistors illustrated and described herein, as well as inassociation with other transistors and structures not illustrated,including but not limited to NMOS and/or PMOS composite transistorsformed in the core region and the logic region of an SRAM array.

The method 600 begins at 602, wherein transistor fabrication isinitiated by the formation of gate structures in logic and core regionsof an SRAM device, wherein transistor well formation and isolationprocessing may be initially performed. Act 610 may thus comprisedefining NMOS and PMOS regions, wherein NMOS regions comprise a P-wellin which n-type source/drain regions will later be formed, and PMOSregions comprise an N-well in which p-type source/drain regions willlater be formed, respectively. In addition, isolation regions maycomprise shallow trench isolation (STI) or field oxide regions (FOX)that serve to define various active areas and electrically isolatevarious active areas laterally from one another.

Act 610 may further comprise forming a gate oxide layer in active areasdefined by the various formed isolation regions. In one example, thegate oxide comprises a thin, thermally grown silicon dioxide layer,however, other type gate dielectrics (such as high-k dielectrics) may beformed and are contemplated by the present invention. A conductive gatelayer is then deposited over the gate oxide and is patterned to form aconductive gate electrode. For example, a polysilicon layer may bedeposited via chemical vapor deposition (CVD) and patterned via etchingto form gate electrodes in both NMOS and PMOS regions, respectively.

An offset spacer is then formed on lateral edges of the conductive gateelectrodes also at 610. For example, a thin offset layer (e.g., an oxideor nitride layer) is formed generally conformally over the patternedgate and then etched using a generally anisotropic dry etch to removeoffset layer material on top of the gate and in the source/drainregions, leaving a thin offset spacer material on lateral edges of thegate. The offset spacer, as will be further appreciated below, isemployed in this example to space away the strain inducing materialslightly away from the channel region under the gate, for example, adistance of about 5 nm to about 50 nm.

Optionally at 615, the method may further comprise forming a protectivelayer 720 overlying a portion of the gate structure (e.g., 714 of FIGS.7D, 7E and 7F), for example, in the core 110 and/or logic 120 regions,before the formation of the recesses to prevent polysilicon gate lossand to prevent the formation of a recess in a top portion of the gateelectrode (e.g., 344 of FIG. 4B, 714 of FIGS. 7C-7I) due to a subsequentrecess etch process, for example.

A mask (e.g., 342 of FIGS. 3B-5C) is formed at 620 over both source anddrain regions (e.g., 360 of FIGS. 3B-5C) on opposing sides of thechannel of one or more MOS transistors (e.g., NMOS transistors) in thelogic region (e.g., 120 of FIG. 1) and/or the core region (e.g., 110 ofFIG. 1) of the SRAM device (e.g., 100 of FIG. 1). In one aspect of theinvention, this step is optional.

The mask (e.g., 342 of FIGS. 3B-5C) is also formed at 630 in the moattip regions (e.g., 352 of FIGS. 3B-5C) extending between the gatestructure (e.g., 344 of FIGS. 3B-5C) and the neighboring isolationregion (e.g., 351 of FIGS. 3B-5C), such that only one of the source anddrain regions (e.g., 364 of FIGS. 3B-5C) of one or more MOS transistors(e.g., PMOS transistors) in the core region (e.g., 110 of FIG. 1) ismasked. The moat tip region refers to the active region of the siliconbody which is adjacent to an isolation region (e.g., 351 of FIGS.3B-5C).

A recess is then formed in the unmasked source and drain regions of boththe logic 120 and core regions 110 at 640. The recess is formed using,for example, a dry etching process such as the chemistry employed toetch STI trenches in the semiconductor body when forming isolationregions. The recesses, in one example extend into the semiconductor bodyto a depth of about 10-90 nm, and more preferably about 30-70 nm. In thepresent example, the gate structure is not masked during the recessformation; therefore if the gate electrode is composed of polysilicon, amethod to protect the polysilicon lines during the recess etch may beemployed to prevent polysilicon loss.

The method 600 then continues at 650, wherein silicon germanium (SiGe)is formed in the recesses. In one example, the silicon germanium isformed via a selective epitaxial deposition process such as an LPCVD(low pressure chemical vapor deposition) process using dichlorosilaneand germane as the source gases. In one alternative of the presentinvention, the above reactants are employed to form SiGe in the recessesand subsequently the SiGe is subjected to a p-type implant to form ap-doped SiGe material (e.g., using Boron). Alternatively, and morepreferably, the SiGe is doped in-situ during the selective epitaxialdeposition process by incorporating a p-type dopant reactant in the CVDprocess. For example, diborane or other type reactant may be employed,wherein boron doped SiGe material is formed in the recesses (or otherp-type dopant in the SiGe, as may be appreciated). The in-situ borondoping of the SiGe is preferred because it is believed that the in-situdoped boron is activated to a higher degree than when implanted into theSiGe, and therefore advantageously provides a lower subsequent extensionregion resistance.

While not intending to be limited to any one theory, it is believed thatthe silicon germanium within the recesses form an alloy that has alattice with the same structure as the silicon body lattice, however,the silicon germanium has a larger spacing. Consequently, it is believedthat the silicon germanium within the recesses will tend to expand,thereby creating a compressive stress within the channel of thesemiconductor body underneath the channel.

Thereafter, the selective epitaxial mask (e.g., 342 of FIGS. 3B-5C) andthe polysilicon protective layer (e.g., 720 of FIGS. 7D, 7E and 7F) maybe removed at 660.

Extension regions and source/drain regions may be subsequently formed inthe active region of the silicon body at 670. For example, if the SiGematerial is not doped in-situ as discussed above, an extension regionimplant is then performed, wherein dopants are introduced into thesilicon germanium material in the recesses (both in the moat area aswell as on a top portion of the gate electrode). For example, lightlydoped, medium doped or heavily doped extension region implants areperformed in the NMOS and PMOS regions, or alternatively, the NMOSregions and PMOS regions may be implanted separately with differingdopants by mask off each region, respectively. Since both the recesses(now filled with silicon germanium) and the extension region implantsare formed after the offset spacer, both are self-aligned with respectto the offset spacer, thereby placing both regions extremely close tothe lateral edge of the gate structure within the semiconductor body. Athermal process such as a rapid thermal anneal may then be employed toactivate the extension region dopants, which causes the extensionregions to diffuse laterally slightly underneath the offset spacertoward the channels.

Still referring to FIG. 6, source/drain sidewall spacers are then formedon the gate structures at 670. The sidewall spacers comprise aninsulating material such as an oxide, a nitride or a combination of suchlayers. The spacers are formed by depositing a layer of such spacermaterial(s) over the device in a generally conformal manner, followed byan anisotropic etch thereof, thereby removing such spacer material fromthe top of the gate structure and from the moat or active area andleaving a region on the lateral edges of the gate structure, overlyingthe offset spacers. The sidewall spacers are substantially thicker thanthe offset spacers, thereby resulting in the subsequently formedsource/drain regions to be offset from lateral edges of the gatestructure at least about 50 nm. The source/drain regions are then formedby implantation at 670, wherein a source/drain dopant is introduced intothe exposed areas (top of gate electrode and active areas not covered bysidewall spacers). The source/drain regions are then completed with athermal process to activate the dopant.

The method 600 may then conclude with silicide processing, wherein ametal layer is formed over the device, followed by a thermal process,wherein the metal and silicon interfaces react to form a suicide (on topof the gate and in the source/drain regions). Unreacted metal is thenstripped away, and back end processing such as interlayer dielectric andmetallization layers are formed to conclude the device formation at 680.

The method 600 of the present invention advantageously forms the silicongermanium extremely close to the channel region of the SRAM device inonly and one of the source/drain regions, for example, of the PMOStransistors in the core region of the array, but in both source anddrain regions of the logic region of the SRAM device. Thus, in themethod 600 of the present invention, an asymmetric PMOS transistor isformed having improved drive current and reduced leakage currentcompared to symmetrically formed selective epitaxial growth regions,while avoiding the defects associated selective epitaxial growth in themoat tip regions.

Alternately, in another aspect of the invention, the SiGe is only formedin the source and drain regions of the PMOS transistors of the logicregion but is excluded entirely from the core region of the SRAM device.This variation may be accomplished as above and by masking over (e.g.,covering over with resist) the entire core region of the array. Further,by residing close to the channel, the SiGe more efficiently transfersthe desired stress to the channel. As can be seen, the asymmetric MOStransistor formation and recess location of the present inventionprovides for a substantial improvement in transistor performance.

Consequently, the formation of the recesses prior to the sidewall spacerformation advantageously provides for substantially improved transistorperformance by increasing the electron mobility while mitigating undueformation of threading dislocation defects.

Turning now to FIGS. 7A-7I, a plurality of fragmentary cross sectiondiagrams illustrating PMOS transistors of two neighboring cells of anSRAM memory array device being formed in accordance with the method 600of the present invention of FIG. 6 is provided. In FIG. 7A, PMOStransistors of SRAM cells (e.g., cells 300 and 301 of FIGS. 3B-5C) of anSRAM memory array device 702 is provided, wherein a semiconductor body704, such as a substrate, has one or more wells formed therein, such asa P-well (not shown) to define an NMOS transistor device region, or anN-well 708 to define a PMOS transistor device region 709, respectively.Further, isolation regions 710 such as STI regions 710 are formed in thesemiconductor body 704 to define active area regions 711, as may beappreciated. In FIG. 7B, the SRAM device 702 is illustrated, wherein agate dielectric 712 has been formed, for example, thermally grown SiO₂,over the active areas 711.

Referring to FIGS. 7C and 7D, a conductive gate electrode material(e.g., polysilicon 344) has been deposited and patterned via an etchingprocess 715 to form two gate electrodes 714 of two neighboring cells300, 301 overlying the gate oxide 712. An offset spacer 716 is thenformed on the lateral edges 714 a of the gate electrode (714 of FIG.7D), wherein the offset spacers 716 have a width 716 a of about 10-50nm.

In FIG. 7D, where the gate electrode comprises polysilicon, apolysilicon protective layer 720 is formed overlying the gate structures714 in the logic and core regions (e.g., respectively 120 and 110 ofFIG. 1) to prevent the formation of a recess in the top portion of thesepolysilicon lines. In addition, an epitaxial etch mask 342 is alsoformed overlying the moat tip regions 352 of the active regions 711,extending between the gate structure 714 and the isolation regions STI710. Mask 342 protects the moat tip regions 352 from the subsequentselective epitaxial etch and growth processes, thereby avoiding thedefects otherwise resulting therefrom.

In FIG. 7E, a recess 718 is then formed in the active areas using anetch process 719, wherein the epitaxial etch mask 342, the gateelectrode 714, and isolations areas 710 serve as a mask for the siliconof the N-well 708 in active areas 711. The recess 718 is then formedinto the semiconductor body to a depth 721 of about 10-90 nm, and morepreferably about 30-70 nm, for example. Although the example illustratedherein has the recess 718 formed after an offset spacer 716, such recessmay be formed prior to such a spacer, wherein in such instance therecess 718 are aligned to the lateral edges 714 a of the gatestructures. In such instances, care should be taken to prevent ashorting of the gate to the subsequently formed epitaxial by providingsome type of isolation therebetween.

Turning now to FIG. 7F, a selective epitaxial deposition process 722 isprovided, wherein a silicon germanium material SiGe 724 is formed in therecess 718. Thereafter, the epitaxial etch mask 342 and the polysiliconprotective layer 720 may be removed and optionally, the source/drainextension regions may be formed in source/drain regions on opposingsides of the channel of the MOS transistors in the logic 120 and coreregions 110 of the SRAM device 702.

As highlighted above, the process 722 may comprise an epitaxialdeposition process, wherein a germanium containing source gas such asgermane is added to the silane or dichlorosilane, such that a silicongermanium material is formed in the recesses. Further, in one example,the selective epitaxial process further includes a diborane source gasto provide for the SiGe to be doped with boron in-situ. Alternatively,other p-type source gases may be employed, as may be appreciated.

Alternatively, the SiGe material may be formed in the recess 718, andthe SiGe 724 subsequently doped with a p-type dopant to form a p-typeextension region in the PMOS region. In such case, the PMOS region 709is then masked with a masking material such as photoresist, and anextension region implant is performed to form n-type extension regionsin the NMOS region. A thermal process such as a rapid thermal anneal isthen performed to activate the dopant, wherein a lateral diffusion ofthe extension regions under the offset spacer 716 is achieved.

The p-type extension region mask is then removed, and an n-typeextension region mask is deposited and patterned to cover the NMOSregion (not shown). In FIG. 7G, a p-type extension region implantprocess 734 is then performed to form p-type extension regions 736 inthe PMOS region 709 of SRAM cells 300 and 301, as illustrated.

The n-type extension region mask (not shown) is then removed andsource/drain sidewall spacers are formed over the offset spacers 716 onthe lateral edges of the gate structures. An insulating sidewallmaterial is deposited in a generally conformal manner over the device702 and subsequently subjected to an anisotropic etch to remove theinsulating material on top of the gate 714 and over the active areas711, leaving sidewall spacers 738 in both the NMOS region and the PMOSregion 709, as illustrated in FIG. 7H.

In FIG. 7I, the method concludes with the formation of source and drainregions 742 in the PMOS regions 709 as well as the NMOS regions (notshown). As discussed above in conjunction with the extension regionimplants, the source/drain implants 743 are performed with an NSD mask(not shown) and then a PSD mask (not shown) in order to implant the NMOSregion and the PMOS region 709 separately with n-type and p-type dopant,respectively. As can be seen in FIG. 7I, the source/drain regions 742are self-aligned with respect to the sidewall spacers 738, and thus arelaterally spaced from the extension regions 736 and the SiGe strainregions 724 nearest the channel (underlying the gate 714).

Thereafter, a silicidation process may be performed, wherein a metallayer is deposited, for example, via sputtering, over the device,followed by a thermal process. During the thermal processing, thoseregions where the metal contacts silicon reacts to form a metal silicide(not shown). More particularly, the silicide forms on the source/drainregions 742 and on top of the gates 714. Subsequently, back endprocessing including metallization may be performed to interconnect thevarious transistors, as may be desired.

Thus, the resulting transistors (e.g., PMOS load transistors 320, or322) of the core region 110 are fabricated asymmetrically in accordancewith the method of the invention of FIG. 6, while the transistors of thelogic region 120 are conventionally fabricated symmetrically. Note thatthe silicon germanium stress inducing region 724 is asymmetricallyformed, wherein SiGe 724 is formed on only one side of each of the PMOStransistors of the PMOS region 709 of the core region 110, and that thisSiGe region 724 is substantially closer to the channel than thesource/drain regions 742. As a result, the overall stress inducedunderlying the channel of the PMOS cell transistors of the core region110 is lower, for example, than the PMOS logic transistors of the logicregion 120 of FIG. 1, having selective epitaxial SiGe growth on bothsides of the channel. Further, having the silicon germanium region 724substantially closer to the channel provides for a substantialimprovement over the prior art in that substantially less germanium isneeded to impart the desired compressive stress to the channel.Consequently, with less germanium needed, few threading dislocationdefects are generated, thereby resulting in substantial performanceimprovements over the prior art, for example, reduced leakage.

The asymmetric transistor configuration eliminates the subsurfaceleakage path located deep into the substrate, under the channel, awayfrom the gate control. This is accomplished in the asymmetric transistorconfiguration by having only one of the source and drain regionssubjected to recess and stressor filling, which removes the short pathbetween two adjacent stressor regions that exist under the gate ofsymmetric transistors.

The asymmetric transistor configuration when used to fabricate reduceddrive current PMOS load transistors 220 and 222 at the SRAM core 110,can reduce the risk of write failures and improve a disturb failuremargin on the cell. The weakening of the SRAM PMOS load transistorsrelative to the logic PMOS transistor increases the ratio of the drivecurrent of the NMOS drive transistor 224 and 226 over the drive currentof the PMOS load transistor 220 and 222. This ratio of transistor drivecurrents on the SRAM cell is typically referred as beta ratio. A higherbeta ratio, which is achieved with a PMOS load transistor with reduceddrive current, improves the yield of the SRAM core 110 by reducing writeand disturb failures. The asymmetric transistor configuration canachieve such improvements without sacrificing the drive currentperformance of the PMOS transistors on the logic region 120.

Therefore, the method of the invention protects the moat tip regionsduring the entire selective epitaxial process to avoid the growththinning and suicide pipe formation defects, and reduces the subsurfaceleakage associated with these regions. Accordingly, the systems andmethods of the present invention provide a memory device, for example,wherein the PMOS transistors of an SRAM memory device having core andlogic regions, exhibit improved mobility and drive current due to theapplication of stress to the channel, and reduced defects associatedwith selective epitaxial growth in moat tip regions of the core regionof the device.

In the above manner, the device may be fabricated with optimized PMOStransistors without any strain applied to the channel in NMOS devices ifdesired.

Further, while the invention is generally described above with respectto PMOS transistor fabrication within the core region (e.g., 110 ofFIG. 1) of an array (e.g., 100 of FIG. 1), NMOS and PMOS transistors maybe fabricated concurrently in the logic region (e.g., 120 of FIG. 1) ofthe array.

In addition, while the invention is described above with respect to theuse of germanium to form a silicon germanium lattice structure, thepresent invention contemplates the use of any element that will createan alloy with silicon and serve to impart a compressive stress to thechannel of the PMOS devices, and such alternatives are contemplated asfalling within the scope of the present invention.

Although the invention has been shown and described with respect to oneor more implementations, equivalent alterations and modifications willoccur to others skilled in the art based upon a reading andunderstanding of this specification and the annexed drawings. Theinvention includes all such modifications and alterations and is limitedonly by the scope of the following claims. In addition, while aparticular feature or aspect of the invention may have been disclosedwith respect to only one of several implementations, such feature oraspect may be combined with one or more other features or aspects of theother implementations as may be desired and advantageous for any givenor particular application. Furthermore, to the extent that the terms“includes”, “having”, “has”, “with”, or variants thereof are used ineither the detailed description or the claims, such terms are intendedto be inclusive in a manner similar to the term “comprising.” Also, theterm “exemplary” is merely meant to mean an example, rather than thebest. It is also to be appreciated that layers and/or elements depictedherein are illustrated with particular dimensions relative to oneanother (e.g., layer to layer dimensions and/or orientations) forpurposes of simplicity and ease of understanding, and that actualdimensions of the elements may differ substantially from thatillustrated herein. Additionally, the layers can be formed in any numberof suitable ways, such as with spin-on techniques, sputtering techniques(e.g., magnetron or ion beam sputtering), (thermal) growth techniquesand/or deposition techniques such as chemical vapor deposition (CVD),for example.

1. An asymmetric SRAM memory device, comprising: an SRAM memory chiphaving MOS transistors in a logic region and a core region of thedevice; one or more logic transistors within the logic region of theSRAM device having a semiconductor body of a first conductivity type,and selective epitaxial regions grown on both source and drain regionsof the logic transistors; and one or more memory cell transistors withinthe core region of the SRAM device, the cell transistors having asemiconductor body of the first conductivity type, and having selectiveepitaxial regions grown on only one of the source and drain regions ofthe memory cell transistors.
 2. The SRAM memory device of claim 1,wherein the logic transistors further comprise: a gate structureoverlying the semiconductor body and defining a channel therebelow inthe semiconductor body, the gate structure defining lateral edges;source and drain extension regions of a second conductivity type formedin the semiconductor body on opposing sides of the channel; stressinducement regions formed in the semiconductor body on opposing sides ofthe channel and generally corresponding to the source and drainextension regions; sidewall spacers residing over the lateral edges ofthe gate structure; and source and drain regions of the secondconductivity type formed in the semiconductor body on opposing sides ofthe channel.
 3. The SRAM memory device of claim 1, wherein the memorycell transistors further comprise: a gate structure overlying thesemiconductor body and defining a channel therebelow in thesemiconductor body, the gate structure defining lateral edges; sourceand drain extension regions of a second conductivity type formed in thesemiconductor body on opposing sides of the channel; a stress inducementregion formed in the semiconductor body on only one side of the channeland generally corresponding to one of the source and drain extensionregions; sidewall spacers residing over the lateral edges of the gatestructure; and source and drain regions of the second conductivity typeformed in the semiconductor body on opposing sides of the channel. 4.The SRAM memory device of claim 3, wherein the first conductivity typeis n-type and the second conductivity type is p-type, and wherein thestress inducement regions comprise silicon germanium formed by etchingrecesses in the semiconductor body and forming silicon germanium thereinvia in-situ doped selective epitaxial deposition.
 5. The SRAM memorydevice of claim 1, further comprising offset spacers formed on thelateral edges of the gate structure and disposed between the lateraledges of the gate structure and the sidewall spacers, respectively,wherein the distance between the stress inducement regions is dictatedby a width of the gate structure and the offset spacers.
 6. The SRAMmemory device of claim 1, wherein the MOS transistors of the SRAM devicehaving a semiconductor body of the first conductivity type comprisesPMOS transistors of a 6T SRAM cell.
 7. The SRAM memory device of claim1, wherein the other one of the source and drain regions of the one ormore memory cell transistors within the core region of the SRAM device,generally corresponds to moat tip regions, which are characterized by aregion of interface between a source or drain region moat and aneighboring isolation structure.
 8. An SRAM memory device, comprising:an SRAM memory chip having MOS transistors in a logic region and a coreregion of the device; one or more logic transistors within the logicregion of the SRAM device having a semiconductor body of a firstconductivity type, and selective epitaxial regions grown on both sourceand drain regions of the logic transistors; and one or more memory celltransistors within the core region of the SRAM device having asemiconductor body of the first conductivity type, and having selectiveepitaxial regions grown on neither the source nor the drain regions ofthe memory cell transistors.
 9. The SRAM memory device of claim 8,wherein the logic transistors further comprise: a gate structureoverlying the semiconductor body and defining a channel therebelow inthe semiconductor body, the gate structure defining lateral edges;source and drain extension regions of a second conductivity type formedin the semiconductor body on opposing sides of the channel; stressinducement regions formed in the semiconductor body on opposing sides ofthe channel and generally corresponding to the source and drainextension regions; sidewall spacers residing over the lateral edges ofthe gate structure; and source and drain regions of the secondconductivity type formed in the semiconductor body on opposing sides ofthe channel.
 10. The SRAM memory device of claim 8, wherein the memorycell transistors further comprise: a gate structure overlying thesemiconductor body and defining a channel therebelow in thesemiconductor body, the gate structure defining lateral edges; sourceand drain extension regions of a second conductivity type formed in thesemiconductor body on opposing sides of the channel; sidewall spacersresiding over the lateral edges of the gate structure; and source anddrain regions of the second conductivity type formed in thesemiconductor body on opposing sides of the channel.
 11. The SRAM memorydevice of claim 8, further comprising offset spacers formed on thelateral edges of the gate structure and disposed between the lateraledges of the gate structure and the sidewall spacers, respectively,wherein the distance between the stress inducement regions is dictatedby a width of the gate structure and the offset spacers.
 12. The SRAMmemory device of claim 8, wherein the first conductivity type is n-typeand the second conductivity type is p-type, and wherein the stressinducement regions comprise silicon germanium formed by etching recessesin the semiconductor body and forming silicon germanium therein viain-situ doped selective epitaxial deposition.
 13. A method ofasymmetrically forming MOS transistors of an SRAM memory device having alogic region and a core region, the method comprising: forming a gatestructure over a first type semiconductor body and defining a channeltherebelow in the semiconductor body of the MOS transistors in the logicand core regions of the device; masking only one of a source and drainregions of the MOS transistors in the core region, the masked regionsgenerally corresponding to moat tip regions; forming a recesssubstantially aligned to the gate structure in the semiconductor body inboth of the source and drain regions of the logic region of the device;forming a recess substantially aligned to the gate structure in thesemiconductor body in the other unmasked one of the source and drainregions of the core region of the device; epitaxially growing silicongermanium in the recesses; and removing the mask.
 14. The method ofclaim 13, further comprising forming offset spacers on the lateral edgesof the gate structure before forming the recesses, wherein the recessesare aligned in the semiconductor body with respect to the offsetspacers.
 15. The method of claim 13, further comprising masking overboth of the source and drain regions on opposing sides of the channel ofthe MOS transistors in the core region.
 16. The method of claim 13,further comprising forming source and drain extension regions in thesemiconductor body in the source and drain regions on opposing sides ofthe channel of the MOS transistors in the logic and core regions of thedevice after removing the mask.
 17. The method of claim 13, whereinepitaxially growing the silicon germanium comprises performing aselective epitaxial deposition of silicon germanium in the presence of ap-type dopant containing source gas, wherein the p-type dopant dopes theepitaxially growing silicon germanium in-situ.
 18. The method of claim13, further comprising: forming sidewall spacers over lateral edges ofthe gate structure after growing the silicon germanium in the recess;and implanting p-type source and drain regions in the semiconductor bodyafter forming the sidewall spacers.
 19. The method of claim 13, furthercomprising implanting p-type extension regions into the silicongermanium before forming the sidewall spacers.
 20. The method of claim13, further comprising: forming offset spacers on the lateral edges ofthe gate structure before forming the recesses, wherein the recesses arealigned in the semiconductor body with respect to the offset spacers;performing a p-type extension region implant after formation of theoffset spacers and before forming the recesses; and performing a thermalprocess after the extension region implant and before forming therecesses, wherein the extension regions slightly diffuse laterally inresponse thereto such that edges thereof extend below the offset spacerstoward the gate structure.
 21. The method of claim 13, furthercomprising forming a protective layer overlying a portion of the gatestructure before the formation of the recesses to prevent the formationof a recess in the gate electrode.
 22. The method of claim 21, furthercomprising removing the gate structure protective layer after theepitaxial growth of silicon germanium in the recesses associated withthe semiconductor body.
 23. A method of asymmetrically forming MOStransistors of an SRAM memory device having a logic region and a coreregion, the method comprising: forming a gate structure over a firsttype semiconductor body and defining a channel therebelow in thesemiconductor body of the MOS transistors in the logic and core regionsof the device; masking only one of a source and drain regions of the MOStransistors in the core and logic regions; forming a recesssubstantially aligned to the gate structure in the semiconductor body inthe other unmasked one of the source and drain regions of the core andlogic regions of the device; epitaxially growing silicon germanium inthe recesses; and removing the mask.
 24. The method of claim 23, furthercomprising forming offset spacers on the lateral edges of the gatestructure before forming the recesses, wherein the recesses are alignedin the semiconductor body with respect to the offset spacers.
 25. Themethod of claim 23, further comprising forming source and drainextension regions in the semiconductor body in the source and drainregions on opposing sides of the channel of the MOS transistors afterremoving the mask.
 26. The method of claim 23, wherein epitaxiallygrowing the silicon germanium comprises performing a selective epitaxialdeposition of silicon germanium in the presence of a p-type dopantcontaining source gas, wherein the p-type dopant dopes the epitaxiallygrowing silicon germanium in-situ.
 27. The method of claim 23, furthercomprising: forming sidewall spacers over lateral edges of the gatestructure after growing the silicon germanium in the recess; andimplanting p-type source and drain regions in the semiconductor bodyafter forming the sidewall spacers.
 28. The method of claim 27, furthercomprising implanting p-type extension regions into the silicongermanium before forming the sidewall spacers.
 29. The method of claim23, further comprising: forming offset spacers on the lateral edges ofthe gate structure before forming the recesses, wherein the recesses arealigned in the semiconductor body with respect to the offset spacers;performing a p-type extension region implant after formation of theoffset spacers and before forming the recesses; and performing a thermalprocess after the extension region implant and before forming therecesses, wherein the extension regions slightly diffuse laterally inresponse thereto such that edges thereof extend below the offset spacerstoward the gate structure.
 30. The method of claim 23, furthercomprising forming a protective layer overlying a portion of the gatestructure before the formation of the recesses to prevent the formationof a recess in the gate electrode.
 31. The method of claim 30, furthercomprising removing the gate structure protective layer after theepitaxial growth of silicon germanium in the recesses associated withthe semiconductor body.